You said "The customer says that one of the thermostats reads a temperature of 70 deg. F even though he has it set at 60 deg. F. This thermostat is in a single gang box with two heating elements run to it. The heating elements are imbedded in the ceiling plaster. This is the only thermostat that has two elements run to it. The others have only one element."
Just for clarity the stat is set at 60 and the temp reading is 70? Is the heat on or off when it is showing 70? If the heat is off then there is no problem unless the stat also controls cooling. If the heat is on and it is 70 degrees in the room there is either a calibration problem or the stat is broke. From your customers statement it appears he thinks the room temperature should be 60 even if it is warmer outside. There can be lots of reasons the room is warmer than the stat setting without there being a problem with the heating. Now if the stat is set at 70 and it is 60 in the room that is a heating problem otherwise he has an AC problem.
